This invention is concerned with complex, matched metal die molded parts of essentially glass reinforcement fibers embedded in a matrix of essentially thermosetting synthetic resins. The material system is commonly referred to as sheet molding compound (SMC). Such parts are commonly utilized as automotive component panels, e.g., roof panels, deck and hood lids, and the like.
Such panels necessarily have incorporated therein integrally formed ribs, bosses, or other structural reinforcements or fastening locations, and the term "rib" as herein used is intended to generically indicate such reinforcements and locations. The formation of integral ribs on the underside of the exposed surface of such a panel necessarily requires an abrupt change in the thickness of the molded part. In order that the part can be acceptable for automotive use, a blemish-free exposed surface is required, and any interruption or even the most minute surface depression in the panel surface, i.e., one mil or less in depth, will render the panel unacceptable for automotive use. The term "sink" utilized herein to define such localized depression in the surface continuity of the panel.
Generally, sink is the result of the designed cross-sectional change inherent in the formation of such panels. It is generally accepted that the two major causes of sink in thermoset plastic moldings are polymerization shrinkage and thermal shrinkage.
Polymerization shrinkage is the volumetric change which occurs when polyester resins or other thermoset resins are polymerized and cross-linked. This type of shrinkage can be minimized by the utilization of "low-profile" additives, as is well known in the art. Generally, some type of thermoplastic, such as styrene, which is soluble in the polyester monomer is added to the polyester for co-polmerization to form a discontinuous phase as the polyester is cured. By the use of such low-profile resin systems, polymerization shrinkage is virtually eliminated as a cause of "sink" formation.
Thermal shrinkage may be of several different types. Overall thermal shrinkage is due to the conditions under which the parts is molded, e.g., the difference in temperature between the as-molded panel and the ambiant atmosphere, the time and pressure of molding, etc. Secondly, thermal shrinkage can occur locally between different portions of the molded panel, and this factor can be minimized by the geometry of the part. The larger the mass of material in the rib or boss, the greater the temperature differential that occurs when the resin cross-links. Thus, it is desirable to use thin ribs. These two factors of overall thermal shrinkage and localized thermal shrinkage and the use of low-profile resins are all known in the prior art as explained in a paper presented by J. J. McCluskey and R. Z. Bell to the Twenty-Sixth Annual Conference of the Society of the Plastics Industry on February 9-12, 1971, at Washington, D.C.